Limit power losses by using a synchronous buck LED driver as a boost mode converter

Synchronous buck converter drivers are commonly used when the current required for high power LEDs, such as 10–40A projector LEDs, would overstress the components in a nonsynchronous converter. Synchronous rectification limits power losses and thermal rise due to high current in the converter switches. Synchronous rectification can offer the same benefits in high power step-up (boost) LED drivers - even with 1 to 3A LEDs. In contrast to a buck converter, the peak switch current of a boost can be much higher than the LED current, especially when output power is high and the input voltage is low.

There are a number of situations where a synchronous boost LED driver is not available for a particular application. For some of these cases, a synchronous buck LED driver IC can be used, but instead of operating as a step-down converter, it operates as a step-up, or boost mode LED driver.

For instance, take the LT3744 40V synchronous buck LED driver, which is designed to drive high current LEDs for projectors. It features a versatile, floating V_EE output that allows it to be used in both high current buck applications and positive-to-negative (buck-boost) topologies where the anode of the high current LED is connected to ground to satisfy heat-sinking requirements. It is the floating V_EE feature that allows us to effectively use this part, originally designed for buck applications, as a synchronous boost mode LED driver.

Figure 1 - Boost mode 9–16V input to 25V, 3A LED driver with 98% efficiency

LT3744 boost mode LED driver

The LT3744 synchronous boost mode LED driver shown in Figure 1 regulates a 3A, 25V (75W) LED string from an automotive input (9–16V) at 98% efficiency. Even at this power level, the maximum component temperature rise is 45°C with a 12V input, as shown in Figure 2. The IC enables easy implementation of both 10:1 analogue and 100:1 PWM dimming at 120Hz with ground-referred input signals, even though neither the LED string nor the PWM dimming MOSFET are connected to GND.

Figure 2 - Boost mode LED thermal scan shows cool operation

Although the 5mΩ sense resistor sets a 10A peak switch current in this application, the solution can be altered to operate with a 6V input and a 15A peak switch current; with an appropriately valued inductor and a lowered undervoltage lockout.

The negative V_EE rail on the LT3744 can reach −21V. The LT3744 simplifies design by handling the level-shifting of ground referenced input control signals. A simple ground referenced PWM input signal is level shifted to PWM_OUT, so no additional level shifting circuitry is required to control the PWM MOSFET. Likewise, the LED current-setting sense resistor can be tied directly to the negative V_EE rail, all the way down to −21V.

Level-shifted GND and V_EE

The level-shifting, positive-to-negative conversion, feature of the LT3744 LED driver is designed to support high current grounded-anode LED drivers. Nevertheless, the same feature can be used for boost mode applications where the LED string is connected between V_IN and a negative V_EE potential. Because the LED sense resistor and PWM dimming MOSFET are both located at the bottom of the LED string, the level-shifted PWM_OUT signal from the ground-referred PWM input yields a topology that looks no more complicated than a traditional boost PWM dimming setup. The input side looks like a straightforward LED driver, with CTRL analogue dimming, SYNC input and enable inputs all referred to signal GND, no matter where negative V_EE lies.

The LT3744’s V_EE can go all the way down to −21V. Open LED overvoltage protection is set at about 26.5V for the 25V V_LED application. With some open LED overshoot in mind, this limits the V_IN minimum to about 6V for a 25V V_LED boost mode application before negative V_EE goes beyond the −21V limit. To operate at the minimum 6V input, the solution in Figure 1 requires a lower undervoltage lockout and a sense resistor and inductor that can accommodate 15A peak switch current limit. With lower V_LED strings (at any current level), the minimum input voltage can be dropped to 4V_IN with some simple adjustments.

Figure 3 - Boost mode current ripple, duty cycle and voltage stress are the same as those of traditional boost regulators

More about boost mode

A boost mode converter has many of the same properties of traditional boost regulators. As shown in Figure 3, with the exception of the unusual topological hookup and the main control of high side switch S1 instead of a low side switch, this boost mode converter features the same duty cycle, ripple current and voltage stress that a tradition boost converter would. If synchronous rectification is not required, a nonsynchronous buck regulator can be used as a boost mode driver, with S2 replaced by a typical catch diode D1, as in a traditional boost regulator.

The gain-phase Bode plot in Figure 4 shows that even the control loop of boost mode converter acts like a traditional boost regulator. With a 10kHz crossover frequency, 60° of phase margin, and −15dB of gain margin, the LED driver shown in Figure 1 is stable and reliable.

Figure 4 - Boost mode control loop gain and phase shows typical bandwidth

Conclusion

Synchronous rectification is a feature commonly called on to limit power losses in high power LED drivers. As shown here, a synchronous buck LED driver can be used as boost mode converter, capitalising on the wide availability of this feature in high performance buck regulator ICs. Specifically, the LT3744 synchronous buck LED driver can be used as a high efficiency 9V–16V input, 25V_LED 3A boost mode LED driver, yielding 98% efficiency for a 75W converter. The unique ability of this IC to level shift control signals as needed from SGND to –V_EE makes it possible to produce a floating boost mode topology with no more components than needed in a traditional boost.